1. Field of the Invention
This invention relates to a method of preventing catalyst deactivation in the conversion of aromatic hydrocarbons particularly in a hydrogen atmosphere, by contacting the catalyst with hydrocarbons under controlled conditions in advance of start-up.
2. Description of the Prior Art
Silica-aluminas or zeolites have been utilized widely for alkylation, dealkylation, disproportionation and isomerization of aromatic hydrocarbons. However, these catalysts for the conversion of aromatic hydrocarbons have the disadvantage that their activity gradually declines due to the deposition of carbonaceous material and resulting clogging of the pore structure, especially in zeolite that has strong acid sites and small pore structure.
In order to overcome this disadvantage, methods have been proposed for preventing catalyst deactivation by partially poisoning a zeolite catalyst with a metal, by sulfiding a zeolite catalyst and, further, continuously introducing a sulfur compound into the reaction system. It has also been suggested to effect the reaction at a low temperature for the purpose of preventing deposition of carbon but the object of effecting simultaneously both the reactions of disproportionation of ethylbenzene and isomerization of xylene, for example, cannot be accomplished since the disproportionation of ethylbenzene takes place preferentially over the isomerization of xylene.
Various patent publications disclose processes wherein aromatic hydrocarbons, in some cases in the absence of hydrogen, are passed in the liquid phase over catalysts. However, such processes are not for the pretreatment of the catalysts but are for the conversion itself of the aromatic hydrocarbons by the reactions mentioned above. They do not employ the particular catalyst of this invention.
In U.S. Pat. No. 3,377,400, issued on Apr. 9, 1968 to J. J. Wise (assigned to Mobil Oil Corporation) a method is disclosed for subjecting alkyl aromatic hydrocarbons to isomerization or disproportionation by contact with a crystalline aluminosilicate, in the liquid phase, at temperatures below 600.degree. F.
In U.S. Pat. No. 3,578,723 issued on May 11, 1971 to Bowes and Wise (assigned to Mobil Oil Corporation) crystalline zeolites identified as ZSM-4 are contacted with alkyl aromatics at a temperature in the range of 250.degree. F. to 1000.degree. F. at a pressure of up to 2000 psig at a liquid hour space velocity of 0.05 to 40 to effect rearrangements including disproportionation and isomerization.
In U.S. Pat. No. 3,856,873 issued on Dec. 24, 1974 to G. T. Burress (assigned to Mobil Oil Corporation) certain acid zeolites are brought into contact with C.sub.8 aromatics to isomerize xylenes and convert at least part of the ethyl benzene to other aromatics, in the vapor phase, in the absence of added hydrogen, at a temperature of 500.degree. to 1000.degree. F.
On the contrary, the object of the present invention is to effect a pretreatment of a catalyst whereby its stability will be improved when it is subsequently used in the conversion of C.sub.8 aromatic hydrocarbons.